Method of driving liquid crystal display device

ABSTRACT

A method of driving a liquid crystal display device of IPS mode is disclosed. The liquid crystal display device includes a pair of substrates, a liquid crystal layer placed between the substrates, gate lines placed above one of the substrates, source lines placed to cross the gate lines with an insulative layer interposed therebetween, a switching element placed near the crossing point of the gate lines and the source lines, and a pixel electrode connected to the source lines through the switching element. A signal voltage required for image display is supplied to the pixel electrode by the source line through the switching element. The method sets an average value of the signal voltage in such a way that an average value of a positive polarity voltage and a negative polarity voltage of the pixel electrode varies with a grayscale to be displayed, and inputs the average value of the signal voltage to the pixel electrode.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of driving an active matrixliquid crystal display device of in-plane switching mode.

2. Related Background Art

Conventional liquid crystal display devices undesirably produce imagesticking, in which, when leaving an unchanging image on a screen for along period of time, a faint remnant of the image remains even after anew image has replaced it. A driving method used to overcome thisproblem is offset correction to correct the grayscale dependence ofvoltage drop induced by a gate signal. Another driving method forreducing the image sticking and eliminating flicker in a twisted nematic(TN) liquid crystal display device is as follows. In a grayscale where asource signal has a large amplitude, the voltage of a common signal andthe center voltage of the source signal are set to such values as tocompensate the voltage drop inducted by a gate signal. In a grayscalewhere the source signal has a small amplitude, on the other hand, thecenter voltage of the source signal is set to a value higher than theabove center voltage of the source signal for compensating the voltagedrop induced by the gate signal. This method is disclosed in JapaneseUnexamined Patent Application Publication No. 2001-337310, pp. 2–5, andillustrated in FIG. 8–9, for example.

However, if the driving method described in the above application isapplied to a liquid crystal cell of in-plane switching (IPS) mode, it isunable to prevent the image sticking. The liquid crystal cell of the IPSmode has electrodes parallel to each other, which is different from thatof the TN mode having electrodes opposite to each other, and a residualDC voltage is more likely to be retained in the IPS mode than in the TNmode, causing the image sticking.

SUMMARY OF THE INVENTION

In view of the foregoing, it is an object of the present invention toprovide a method of driving a liquid crystal display device capable ofreducing image sticking in a liquid crystal display device of IPS mode,not by a conventional offset correction of setting an optimum value ofVcom for preventing unequal DC components between a pixel electrodevoltage and a common electrode voltage, but by setting an offset valuefor preventing residual vertical DC voltages and image sticking.

To these ends, according to one aspect of the present invention, thereis provided a method of driving a liquid crystal display device ofin-plane-switching (IPS) mode for applying an electric fieldsubstantially parallel to a substrate surface, having a pair ofsubstrates, a liquid crystal layer placed between the pair ofsubstrates, a plurality of gate lines placed above one of the pair ofsubstrates, a plurality of source lines placed to cross the gate lineswith an insulative layer interposed therebetween, a switching elementplaced in a near proximity to a crossing point of the gate lines and thesource lines, and a pixel electrode connected to the source linesthrough the switching element, to which a signal voltage required forimage display is supplied by the source line through the switchingelement, the method including a step of setting an average value of thesignal voltage in such a way that an average value of a positivepolarity voltage and a negative polarity voltage of the pixel electrodevaries with a grayscale to be displayed, and a step of inputting theaverage value of the signal voltage to the pixel electrode. This methodis capable of minimizing image sticking in IPS liquid crystal displaydevices.

In the above method, the average value of the signal voltage may be anaverage of two values of grayscale reference voltages input to a sourceline drive circuit for generating the signal voltage from an externalunit.

According to another aspect of the present invention, there is provideda liquid crystal display device of in-plane-switching (IPS) mode forapplying an electric field substantially parallel to a substratesurface, including a pair of substrates, a liquid crystal layer placedbetween the pair of substrates, a plurality of gate lines placed aboveone of the pair of substrates, a plurality of source lines placed tocross the gate lines with an insulative layer interposed therebetween, aswitching element placed in a near proximity to a crossing point of thegate lines and the source lines, and a pixel electrode connected to thesource lines through the switching element, to which a signal voltagerequired for image display is supplied by the source line through theswitching element, wherein an average value of the signal voltagesupplied by the source line is set in such a way that an average valueof a positive polarity voltage and a negative polarity voltage of thepixel electrode varies with a grayscale to be displayed. This IPS liquidcrystal display device is capable of minimizing image sticking.

In the above liquid crystal display device of in-plane-switching mode,the average value of the signal voltage may be an average of two valuesof grayscale reference voltages input to a source line drive circuit forgenerating the signal voltage from an external unit.

The above and other objects, features and advantages of the presentinvention will become more fully understood from the detaileddescription given hereinbelow and the accompanying drawings which aregiven by way of illustration only, and thus are not to be considered aslimiting the present invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1A is a view showing a liquid crystal display device according toan embodiment of the present invention.

FIG. 1B is a cross sectional view of the liquid crystal display devicein FIG. 1A.

FIG. 2 is a circuit diagram of a liquid crystal display device accordingto an embodiment of the present invention.

FIG. 3 is a view showing a signal waveform of a liquid crystal displaydevice according to an embodiment of the present invention.

FIG. 4 is a view showing a relationship between a capacitance and asignal voltage amplitude in a liquid crystal display device according toan embodiment of the present invention.

FIG. 5 is a circuit diagram of a liquid crystal display device accordingto an embodiment of the present invention.

FIG. 6A is a sectional view of a major part of a liquid crystal displaydevice of IPS mode.

FIG. 6B is a sectional view of a major part of a liquid crystal displaydevice of TN mode.

FIG. 7A is a view showing a polarity inversion driving method in aliquid crystal display device according to an embodiment of the presentinvention.

FIG. 7B is a view showing a polarity inversion driving method in aliquid crystal display device according to an embodiment of the presentinvention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

A transmissive liquid crystal display device applies an electric fieldto liquid crystal filled between two substrates and controls thealignment of liquid crystal molecules according to the strength of theelectric field, thereby adjusting the amount of light passing throughthe substrate to produce an image with a desired brightness. In anactive matrix liquid crystal display device with a switching elementcomprising a thin film transistor (TFT) and so on, in-plane switching(IPS) mode that applies an electric field parallel to a substrate toliquid crystal is used as a technique to obtain an extremely wideviewing angle. The IPS mode operation can minimize viewing-angle-basedgrayscale inversion and deterioration in contrast ratio.

Referring first to FIG. 1A, a plan view of a pixel area of a standardIPS liquid crystal display device is shown. FIG. 1B is a sectional viewthereof. The liquid crystal display device has a TFT array substrate 100and a color filter substrate 200.

The TFT array susbtrate 100 has a plurality of gate lines 1 placed on aninsulative substrate 101, an insulative layer 2 placed over the gatelines, a source line 3 placed to cross the plurality of gate lines 1,and an insulative layer 4 placed over the source line 3. The gate lines1 and the source line 3 are connected to a switching element such as TFTplaced at their crossing point. A V-shaped comb-type pixel electrode 6consisting of a plurality of electrodes placed parallel to the sourceline 3 is connected to the switching element 5. A V-shaped comb-typecommon electrode 7 also consists of a plurality of electrodes, placedparallel to and alternating with the plurality of electrodes of thepixel electrode 6. The pixel electrode 6 is formed by a metal such aschromium (Cr), aluminum (Al), and molybdenum (Mo), or a transparentconductive film such as Indium Tin Oxide (ITO). A storage capacitor line8, formed by a metal such as chromium (Cr), aluminum (Al), andmolybdenum (Mo), is connected to the common electrode 7 through athrough-hole. The TFT array substrate 100 and the color filter substrate200 are placed face to face and a liquid crystal layer 9 is interposedbetween, which makes up a liquid crystal panel 10. Further, a backlightand other components are set to the liquid crystal panel 10, therebyproducing a liquid crystal display device. A voltage is applied to theliquid crystal layer 9 between the pixel electrode 6 and the commonelectrode 7, and an electric field substantially parallel to thesubstrate is thus applied to the liquid crystal layer 9.

In a standard liquid crystal display device of the IPS mode, an activearea of the liquid crystal is surrounded by lamination of dielectricfilms, such as an alignment layer (PI) 61, an protective layer (OC) 62,a color material of a color filter (CF) 63, a glass material of aninsulative substrate 101, and so on, as shown in FIG. 6A. An electricfield E for driving the liquid crystal, created between the pixelelectrode 6 and the common electrode 7, extends into the laminatedfilms. In the liquid crystal display device of the TN mode, on the otherhand, the pixel electrode 6 and the common electrode 7 are placed faceto face, and an electric field E for driving the liquid crystal iscreated only between the pixel electrode 6 and the common electrode 7,as shown in FIG. 6B. Hence, though the TN mode can avoid image stickingby preventing DC component between the pixel electrode 6 and the commonelectrode 7, the IPS mode cannot eliminate the image sticking by theconventional offset correction method due to a residual DC voltage leftin the dielectric lamination films.

Referring then to FIG. 2, a circuit diagram of a liquid crystal displaydevice of the IPS mode is shown. As shown in FIG. 2, a plurality of thehorizontal gate lines 1 and a plurality of the vertical source lines 3cross perpendicularly. The area surrounded by the gate line 1 and thesource line 3 is one pixel area. The switching element 5 is placed nearthe crossing point of the two lines. The pixel electrode 6 is connectedto a drain electrode 23 and placed parallel to the common electrode 7.The source lines 3 are connected to a source line drive circuit 11 forsignal voltage supply. The source line drive circuit 11 supplies agrayscale signal (signal voltage) in accordance with a pixel to bedriven to the liquid crystal panel 10. A gate line drive circuit 12supplies a gate voltage for turning on/off the switching element 5. Acommon voltage is supplied to the common electrode 7.

The circuit shown in FIG. 2 has capacitance Clc with liquid crystalbetween the pixel electrode 6 and a counter electrode, capacitance Cgdbetween the gate and the drain, and storage capacitance Cst. The commonelectrode 7 is kept at a constant common voltage. On the other hand, asignal voltage 32 is written to the pixel electrode 6 from the switchingelement 5 through the drain electrode 23, thus controlling an electricfield in the liquid crystal layer 9 and displaying images.

The operation of a voltage of pixel electrode (hereinafter as a pixelvoltage 31) will be explained hereinafter with reference to FIG. 3showing the waveforms of the voltages applied to the gate line 1 and thesource line 3. In a standard liquid crystal display device, the relativepolarity of the pixel voltage 31 to the common voltage 34 is inverted atevery frame so as to avoid deterioration of liquid crystal. Thus, thepolarity of the pixel electrode 6 relative to the common voltage 34 isinverted at every frame, a time period in which all the gate lines 1 areselected sequentially.

If a positive pulse with a higher voltage than a threshold voltage ofthe switching element 5 is applied to the gate electrode 21, theswitching element 5 turns on, that is, it turns to a high level, inwhich a source electrode 22 and the drain electrode 23 have electricalcontinuity. The signal voltage 32 on the source line 3 is thereby sentto the pixel electrode 6. The signal voltage 32 is an alternatingvoltage with the center voltage of Vso and the amplitude of Vsa. Theamplitude Vsa is in accordance with the grayscale to be displayed. Thepixel voltage 31 rises in synchronization with the gate voltage 33, asshown in FIG. 3. Upon switching the gate voltage 33 to a low level toturn off the switching element 5, feedthrough of ΔVgd occurs in thepixel voltage 31 due to the capacitance Cgd between the gate and thedrain. For one frame period after that, the pixel voltage 31 is retainedby the storage capacitance Cst. The common voltage 34 is set so as toequalize the absolute value of a voltage V1 applied to the liquidcrystal in the first frame period and that of a voltage V2 appliedthereto in the second frame period, and the common voltage 34 of thisvalue is called an optimum Vcom. The common voltage 34 maybe adjustedwith a variable resistor in such a way that the absolute values of V1and V2 are the same.

A voltage drop ΔVgd of the pixel voltage 31 due to the feedthrough isexpressed by:ΔVgd=ΔVg×Cgd/(Clc+Cgd+Cst)  Formula 1where ΔVg is a change in the gate voltage.After the pixel voltage 31 is kept at a constant value for one frameperiod, in the second frame period, upon the gate voltage 33 turningagain to the high level, the pixel voltage 31 turns to a level of thesignal voltage 32 with reverse polarity to the previous (first) frameperiod. Then, in synchronization with the gate voltage 33 turning backto the low level, the pixel voltage 31 drops as in the first frameperiod, and further drops by ΔVgd due to the feedthrough.

Of the elements of Formula 1, Clc is dependent on the signal voltage(grayscale voltage). The value of the capacitance Clc with the liquidcrystal thus varies according to the grayscale of image applied to theliquid crystal. Cst and Cgd have little voltage dependence.

Referring now to FIG. 4, a relationship between Clc and amplitude Vsa ofthe signal voltage 32 is shown. As the signal voltage 32 increases, Clsincreases, and accordingly ΔVgd decreases in inverse proportion to thesignal voltage 32. Thus, ΔVgd is not constant but varies with theamplitude of the signal voltage 32, as expressed by Formula 1. Hence, inorder to equalize the absolute value of the voltage V1 applied in thefirst frame period and that of the voltage V2 applied in the secondframe period in each grayscale, it is necessary to set the value of Vsoin accordance with changes in ΔVgd. This is called the offsetcorrection. TN liquid crystal display devices generally prevent theimage sticking with this method. Another method to prevent the imagesticking is to set a high value for Vso at the grayscale with small_Vsa,as described above. This embodiment sets a value of Vso for preventingthe image sticking in the IPS liquid crystal display device withoutusing the above method of the Vso setting.

A method of setting Vso will be explained below. Referring to FIGS. 2and 3, a control circuit 13 controls the output timing and the value ofthe gate voltage 33 output from the gate line drive circuit 12, thesignal voltage 32 output from the source line drive circuit 11, and thecommon voltage 34 applied to the common electrode 7. For example, with256 levels of grayscale reproduction, a given value of the signalvoltage 32 selected from 256 levels, positive and negative polaritieseach, of voltages is applied to the pixel electrode so as to reproducethe grayscale. Thus, it is necessary to output the signal voltages 32 of256 different levels, positive and negative polarities each, from thesource line drive circuit 11.

Now, a unit of generating 256 levels of the signal voltages 32 will beexplained below. Referring to FIG. 5, it is an explanatory view showingan example of the source line drive circuit 11, a unit of generating 256levels of the signal voltages. As shown in FIG. 5, there are provided aninput terminal 41 of the source line drive circuit 11 from a DC powersource 14, an input terminal 42 of the source line drive circuit 11 fromthe control circuit 13, a divided resistor 43, and an output terminal 44of the source line drive circuit 11. The input terminal 42 of the sourceline drive circuit 11 is connected to the control circuit 13 and controlsignals are input to the input terminal 42. The input terminal 41 of thesource line drive circuit 11, on the other hand, is connected to the DCpower source 14 and grayscale reference voltages of 16 different levels(values) are input the input terminal 41. For example, 16 levels ofgrayscale reference voltages, which are, the first grayscale referencevoltage (Vref0), the second grayscale reference voltage (Vref1), to thefifteenth grayscale reference voltage (Vref14), and the sixteenthgrayscale reference voltage (Vref15), are applied, respectively, to theinput terminals 41 from output terminals of the DC power source 14.Divided resistors 43 connected in series are connected between the inputterminals 41.

For example, 15 divided resistors 43 are connected between the inputterminal 41 with the first grayscale reference voltage (Vref0) and theinput terminal 41 with the second grayscale reference voltage (Vref1).From output terminals 45 placed at each connection of the dividedresistors 43, 15 signal voltages of different levels, which are, thesignal voltages indicating the first screen voltage (Vs0), the secondscreen voltage (Vs1) to the sixteenth screen voltage (Vs15), divided bythe 15 divided resistors are respectively output. With the signalvoltages between all Vref, it is able to generate 256×2 levels of signalvoltages; 256 levels at the positive polarity side and 256 levels at thenegative polarity side. A given voltage is selected from those signalvoltages in a selection circuit 46 and output to the liquid crystalpanel 10 through the output terminal 44.

Table 1 shows the grayscale reference voltage Vref input to the sourceline drive circuit 11, and the amplification Vsa and the center voltageVso of the signal voltage 32 output from the source line drive voltage11 according to this embodiment. For example, in the case of thegrayscale of 255, the center voltage Vso of the signal voltage 32 inputto the source line drive circuit 11 is 7.435V, which is the average ofthe grayscale reference voltages Vref0=14.670V and Vref15=0.200V. In thegrayscale of 0, Vso is 7.355V, the average of Vref7=7.985V andVref8=6.725V. These set values are determined by a test of examining thedegree of image sticking with varying Vref. The set values may beadjusted according to the specifications of the liquid crystal displaydevice. This method can prevent DC component from remaining in thedielectric lamination films, thereby minimizing the image sticking.

TABLE 1 GRAYSCALE Vsa Vso VOLTAGE VALUE 255 7.235 7.435 Vref0 = 14.670240 6.410 7.313 Vref1 = 13.723 192 5.210 7.135 Vref2 = 12.345 127 4.0006.955 Vref3 = 10.955 64 2.800 7.097 Vref4 = 9.897 32 1.950 7.198 Vref5 =9.148 1 0.700 7.347 Vref6 = 8.047 0 0.630 7.355 Vref7 = 7.985 Vref8 =6.725 Vref9 = 6.647 Vref10 = 5.248 Vref11 = 4.297 Vref12 = 2.955 Vref13= 1.925 Vref14 = 0.903 Vref15 = 0.200

This embodiment shows a case where the difference (Vso255−Vso127)between Vso at the grayscale of 255 (Vso255) and Vso at the grayscale of127 (Vso127) is 480 mV, and the difference (Vso0−Vso127) between the Vsoat the grayscale of 0 (Vso0) and Vso127 is 400 mV. In other cases,however, either or both of these values can be substantially 0 mV ornegative. These values may vary with the combination or the interfacestate of the alignment layer 61, the protective layer 62, and thematerial of the color filter 63.

In the liquid crystal display device according to this embodiment, as instandard liquid crystal display devices, adjustment of Vcom is performedwith an intermediate grayscale image at the grayscale of 127, which isthe intermediate value between the grayscales of 0 and 255. If theliquid crystal display device uses a dot-inversion driving method and soon, it displays a checkered pattern, for example, and adjusts the valueof Vcom in such a way that the pixel area with minimum flicker has anoptimum Vcom. Hence, though substantially no flicker occurs at thegrayscale of 127, flicker is likely to occur at the other grayscalessince their Vcom values are off the optimum Vcom value. The flicker ofimages can be prevented by dot-inversion driving that reverses thepolarity from pixel to pixel as shown in FIG. 7A, 1×2 driving as shownin FIG. 7B, and driving that randomly reverses the polarity, which isnot shown. In FIGS. 7A and 7B, the symbol “+” indicates that a pixelvoltage relative to a common voltage is positive, and the symbol “−”,negative. Though the case of adjusting Vcom at the grayscale of 127 isshown here, the adjustment of Vcom may be performed at the othergrayscales since it has the same effect.

As described above, the present invention provides a method of driving aliquid crystal display device capable of minimizing image sticking in anIPS liquid crystal display device by setting an average value of thesignal voltages in such a way that an average value of a positivepolarity voltage and a negative polarity voltage of the signal voltagessupplied to the pixel electrode varies with grayscale.

From the invention thus described, it will be obvious that theembodiments of the invention may be varied in many ways. Such variationsare not to be regarded as a departure from the spirit and scope of theinvention, and all such modifications as would be obvious to one skilledin the art are intended for inclusion within the scope of the followingclaims.

1. A method of driving a liquid crystal display device ofin-plane-switching mode for applying an electric field substantiallyparallel to a substrate surface, including a pair of substrates, aliquid crystal layer placed between the pair of substrates, a pluralityof gate lines placed above one of the pair of substrates, a plurality ofsource lines placed to cross the gate lines with an insulative layerinterposed therebetween, a switching element placed in a near proximityto a crossing point of the gate lines and the source lines, and a pixelelectrode connected to the source lines through the switching element,to which a signal voltage required for image display is supplied by thesource line through the switching element, comprising: a step of settingan average value of the signal voltage in such a way that an averagevalue of a positive polarity voltage and a negative polarity voltage ofthe pixel electrode varies with a grayscale to be displayed, and a stepof inputting the average value of the signal voltage to the pixelelectrode.
 2. A method of driving a liquid crystal display deviceaccording to claim 1, wherein the average value of the signal voltage isan average of two values of grayscale reference voltages input to asource line drive circuit for generating the signal voltage from anexternal unit.
 3. A liquid crystal display device of in-plane-switchingmode for applying an electric field substantially parallel to asubstrate surface, comprising: a pair of substrates; a liquid crystallayer placed between the pair of substrates; a plurality of gate linesplaced above one of the pair of substrates; a plurality of source linesplaced to cross the gate lines with an insulative layer interposedtherebetween; a switching element placed in a near proximity to acrossing point of the gate lines and the source lines; and a pixelelectrode connected to the source lines through the switching element,to which a signal voltage required for image display is supplied by thesource line through the switching element, wherein an average value ofthe signal voltage supplied by the source line is set in such a way thatan average value of a positive polarity voltage and a negative polarityvoltage of the pixel electrode varies with a grayscale to be displayed.4. A liquid crystal display device of in-plane-switching mode accordingto claim 3, wherein the average value of the signal voltage is anaverage of two values of grayscale reference voltages input to a sourceline drive circuit for generating the signal voltage from an externalunit.